1. Field of the Invention
The invention relates generally to radio engineering, and particularly, to methods of radio signal reception and transmission using adaptive antenna arrays in CDMA (Code Division Multiple Access) cellular communications systems. Also, the present invention can be applied to BTS (Base Transceiver Station) receiving devices that form an antenna pattern for each mobile user in both the reverse and forward channels.
2. Description of the Related Art
The use of an adaptive antenna array in CDMA BTS considerably improves communication quality and system capacity, and expands a BTS coverage area. Consequently, all third generation (3G) standards require use of the adaptive antenna array in a BTS.
The following conventional approaches to form an antenna pattern of a smart antenna in the forward channel are known in the art.
According to a first approach, a weight vector of antenna array elements obtained while receiving a signal in a reverse channel is used for signal transmission in a forward channel. This approach is described in Joseph C. Liberti, Theodore S. Rappaport, “Smart Antennas for Wireless Communication”, Prentice Hall PTR, 1999, and in U.S. Pat. No. 6,031,877 to Simon Saunders, entitled “Apparatus and Method for Adaptive Beam forming in an Antenna Array” granted Feb. 29, 2000; and U.S. Pat. No. 6,122,260 to Hui Liu, Guanghan Xu entitled “Smart Antenna CDMA Wireless Communication System” granted Sep. 19, 2000. The contents of all of the above referenced documents are incorporated herein by reference. This approach is most efficient in TDD (Time Division Duplexing) communication systems.
In TDD systems, the forward and reverse channels are time divided and matched in the carrier frequency. Therefore the direction of the signal traveling from the mobile station (MS) to base station (BS), which is determined by an MS signal, matches a direction of the signal traveling from the BS to the MS. However, for FDD (Frequency Division Duplexing) CDMA systems, the applicability of this method is hardly possible because changes in the carrier frequency might result in considerably different multipath characteristics in the forward and reverse channels.
According to a second approach based on a training signal, an MS estimates forward channel parameters and the estimate is supplied to a BTS via a reverse channel. Based on this estimate, the BTS corrects SA (Smart Antenna) weight factors in the forward channel. This approach is described in article Ayman F. Naguib, Arogyaswami Paulrai, Thomas Kalath. “Capacity Improvement with Base-Station Antenna Arrays in Cellular CDMA”, IEEE Trans. Veh. Technol, vol. 43, no. 3, pp. 691-698, August 1994, and in U.S. Pat. No. 5,828,658 to Bjorn E. Ottersten, Craig H. Barratt, David M. Parish, Richard H. Roy entitled “Spectrally Efficient High Capacity Wireless Communication Systems with Spatio-Temporal Processing” granted Oct. 27, 1998. The contents of all of the above referenced documents are incorporated herein by reference.
The disadvantages of this approach are that a substantial amount of data must be transmitted via the reverse channel to provide feedback and that a long response time of feedback is required. In addition, the use of feedback is impossible in some cellular communications systems, particularly, the 3GPP2 system.
According to a third approach, a direction of arrival of a strongest component of a mobile user multipath signal is determined (see Joseph C. Liberti, Theodore S. Rappaport, “Smart Antennas for Wireless Communication, Prentice Hall PTR”, 1999, and U.S. Pat. No. 6,108,565 to Shimon B. Scherzer entitled “Practical Space-Time Radio Method for CDMA Communication Capacity Enhancement” granted Aug. 22, 2000). This direction is considered as the main direction of a signal traveling from a BTS to an MS. Complex coefficients of the antenna array elements in the forward channel are selected so that the main lobe of the forward channel antenna pattern is oriented in this direction. The width of the main lobe can be determined by an angle sector of the signal.
One possible operation of the third approach is a consequential method disclosed in U.S. Pat. No. 6,108,565. In the method of space-time signal processing a switch beam forming method is used. The width of an antenna beam lobe depends on the distance from mobile users to BTS. If mobile users are in a close vicinity of the BTS, the lobe corresponding to them becomes wider. When mobile users are far away from the BTS, the lobe corresponding to them becomes narrower. Due to the fact that this approach requires information on a distance to the mobile users due to the consequential character of angle spread estimation, it cannot have sufficient accuracy.
A pattern forming method for an adaptive antenna array is described in U.S. Pat. No. 6,108,565, and is the closest to the proposed solution in the prior art (hereinafter referred to as the “prototype”).
The method of utilizing the prototype is described as follows. For each path, weight coefficients of antenna array elements are generated in order to periodically perform the following operations: first, the input signal is demodulated at antenna array elements, then fast Hadamard transformation of the demodulated input signal at the antenna array elements is performed generating the input signal matrix, the input signal matrix is multiplied by the matrix of reference signals, the estimate of the angle of arrival of the input path signal is determined by analyzing the multiplication result of the input signal matrix and matrix of reference signals, the current value of the weight vector is determined as the vector that corresponds to the estimate of the angle of arrival of the input path signal, current values of weight vectors of paths are output and used to determine a pattern of the adaptive antenna array, and the matrix of reference signals is determined by the signals that correspond to pre-determined discrete hypothesizes on the angle of arrival of the input signal.
The estimate of an angle of an arrival of an input signal θ determines a weight vector according to Equation 1 below:w=[1, e−jφ, e−j2φ, K, e−j(N−1)φ]  (1)where
      ϕ    =                            2          ⁢                                          ⁢          π                λ            ⁢      d      ⁢                          ⁢      sin      ⁢                          ⁢      θ        ,λ is a wavelength, d is a distance between antenna array elements, and N is a number of antenna array elements.
In order to implement this method the prototype (conventional) device comprising L path signal processing blocks, which is illustrated in FIG. 1, is used. As illustrated in FIG. 1, the device comprises L path signal processing blocks. Each of L path signal processing blocks contains N parallel channels, consisting of successively connected correlators 2 and fast Hadamrd transformers 3. Also, the device consists of a reference signal generator 1, matrix multiplier and analyzer 4, weight vector of reverse channel antenna array generator 5, and weight vector of forward channel antenna array generator 6.
First inputs of correlators 2.1-2.N are signal inputs and also inputs of the device. The second inputs are reference inputs and combine with the output of reference signal generator 1. The output of each fast Hadamar transformer 3.1-3.N is connected to the corresponding inputs of matrix multiplier and analyzer 4, the output of which is supplied as the input of weight vector of reverse channel antenna array generator 5. A first output of weight vector of reverse channel antenna array generator 5 is the output of the current weight vector of the reverse channel and the first output of the path signal processing block of the device. A second output of weight vector of reverse channel antenna array generator 5 is supplied as the input of weight vector of forward channel antenna array generator 6. The output of weight vector of forward channel antenna array generator 6 is the output of the current weight vector in the forward channel and second output of the path signal processing block of the device.
The prototype (conventional) device illustrated in FIG. 1 operates in the following maimer.
According to the description above of the prototype, in each of L path signal processing blocks, a complex input signal is fed to first (signal) inputs of correlators 2.1-2.N. A reference PN sequence is supplied from reference signal generator 1 to second (reference) inputs of correlators 2.1-2.N. The state of reference signal generator 1 corresponds to the value of the time position of the path signal in the multipath signal to be received. Complex demodulated signals supplied from outputs of correlators 2.1-2.N are fed to inputs of the corresponding fast Hadamard transformers 3.1-3.N, where Hadamard basis decomposition of the input signal is performed. Spectrums of inputs signals supplied from outputs of fast Hadamard transformers 3.1-3.N are supplied to N inputs of matrix multiplier and analyzer 4. In block 4, the matrix of reference signals multiplies the input signal matrix. The inputs signal matrix is generated by the spectrums of input signals. The matrix of reference signals is determined by signals that correspond to predetermined discrete hypothesizes on the angle of arrival of the input path signal.
In addition, in the matrix multiplier and analyzer 4 the multiplication result of the input signal matrix and matrix of reference signals is analyzed and the estimate of the angle of arrival of the input path signal is determined. The estimate of the angle of arrival of the input path signal supplied from the output of the matrix multiplier and analyzer 4 is applied to the input of weight vector of reverse channel antenna array generator 5. Weight vector of reverse channel antenna array generator 5 generates the current weight vector of the reverse channel path at its first output based on the estimate of the angle of arrival of the input path signal. This weight vector is the first output signal of the device.
The estimate of the angle of arrival of the input path signal supplied from the output of weight vector of reverse channel antenna array generator 5 is fed to the input of weight vector of forward channel antenna array generator 6. Weight vector of forward channel antenna array generator 6 generates the current weight vector of the forward channel path at its output based on the estimate of the angle of arrival of the input path signal. This weight vector is the second output signal of the device.
The width of the antenna beam lobe in the forward channel depends on the distance from mobile users to the BTS. If mobile users are in the close vicinity of the BTS, the lobe corresponding to them becomes wider. When mobile users are far away from the BTS, the lobe corresponding to them becomes narrower.
This approach requires determining the distance to mobile users due to the consequential character of angle spread estimation and therefore does not have sufficient accuracy.
Further drawbacks of this method are that in the presence of strong interferences from other users (high rate users, i.e., users with high data transmission rate) the desired signal can be cancelled by the interference and the correct solution about the direction of arrival and angular area of the desired signal cannot be determined.